This invention in general relates to interferometric apparatus and methods and in particular to the construction and use of light sources by which coherent artifacts that would otherwise be present in an interferogram can be suppressed to improve the overall signal to noise ratio.
Optical systems of all types are adversely affected by background light, ghost reflections, and/or unwanted light scattered from elements in an optical assembly, and many techniques have been developed (such as baffles and apertures) to limit the degree to which such undesired light reaches an image. If the optical system uses incoherent light, the background simply adds to the overall light level at the image. In photographic systems, such light may be characterized as veiling glare which operates to reduce contrast in the final photograph. Another common example is the reduction in visibility an automobile driver experiences in viewing through a dirty windshield where scattering operates to generate an overall glare that reduces the contrast in the surrounding landscape.
However, if the optical system uses coherent radiation (e.g., laser light), as is the case with many types of interferometers, scattered light can coherently interfere in the interferometric image to produce large amplitude light level changes with a spatial and/or temporal structure that can completely mask the desired interference pattern. The extreme sensitivity of these interferometers make them adversely affected by even the slightest background that can be produced by the smallest of imperfections in any practical system. Dust or tiny scratches on the optical surfaces of the system, or even variations in the antireflection coatings, are but a few examples of imperfections that can be problematic. Collectively, these flaws are often called optical artifacts, and when observed in coherent optical systems, are known as coherent artifacts.
A commonly used commercial interferometer geometry is known as the Fizeau geometry. The Fizeau geometry has many advantages: the optical system is common path; it has a minimum number of optical components; and is highly manufacturable. However, the unequal path design forces the use of coherent light sources. Hence, light from all locations in the system optics and interferometer, including scattering from small surface defects such as scratches, pits or dust (or volume defects such as bubbles) can influence the interferogram. These defects act as light scattering centers, producing characteristic ring patterns called Newton rings or xe2x80x9cBulls-eyexe2x80x9d patterns that can imprint onto the measured phase map, affecting the extracted surface topography. Even the spurious micro-roughness of good polished surfaces and antireflection coatings contribute to the micro-shape of the wavefronts in the interferometer, and since the wavefronts are no longer common path in such a lateral scale of roughness, they establish themselves in the final measured wavefronts.
One common practice that is responsible for introducing artifacts is the use of commercially available optical components that have not been specifically designed for use in interferometer configurations and light sources with minimization of artifacts in mind yet possess other properties that make their use commercially attractive for economic reasons. Off-the-shelf lenses, for example, often possess desirable performance specifications in terms of aperture, field, focal length, and aberration control, but may have interior structure that, while suitable for other applications, introduce unwanted artifacts in interferometers.
One well-known method for reducing the effects of coherent artifacts in interferometers is to use a spatially extended source, typically in the shape of a disk. However, the spatial coherence of the source is compromised with an extended source resulting in the production of visible interference fringes for only a limited range of interferometer lengths determined by the source diameter.
Accordingly, a primary object of this invention is to describe a new extended source geometry that does not suffer from the contrast degradation of conventional extended sources yet provides excellent suppression of unwanted interference from surfaces and objects far from the object of interest to improve the accuracy and resolution of surface profiling using phase-shifting interferometry.
Another object of the invention is to provide a convenient way to modulate the phase of the interference, further benefiting particular applications developed for phase shifting interferometry.
Yet another object of the invention is to provide a way to suppress interference from surfaces parallel to the surface of interest, such as in the measurement of one surface of parallel flats.
It is yet another object of the invention to provide a source for reducing the effects of artifacts in interferometers using off-the-shelf components.
It is still another object of the invention to provide a source for use in reducing artifacts in unequal path length interferometers such as, for example, Fizeau, Mirau, and Twyman-Green types.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the description to follow is read in conjunction with the drawings.
Generally, the present invention relates to interferometric apparatus and methods for preserving fringe contrast in interferograms while suppressing coherent artifacts that would otherwise be present in the interferogram because of coherent superposition of unwanted radiation generated in the interferometer. Several different embodiments of the invention achieve this result through the use of illumination and interferogrammetric imaging architectures that generate individual interferograms containing the same phase information of preselected characteristics or properties of a test surface (e.g., wavefront, topography) from the perspective of different off-axis points of illumination in an interferometer. Such individual interferograms are combined to preserve fringe contrast while at the same time arranging for artifacts to exist at different field locations so that their contribution in the combined interferogram is diluted. Thus, the same phase differences in the interferometer, corresponding to specific locations on a test surface, are mapped through optics along different light paths.
One embodiment of the invention comprises an illumination mechanism producing an extended source structure, e.g., in the shape of a thin ring of nominally constant radius that is nominally centered around an interferometer system optical axis. The ring defines the interferometer source plane. The interferometer system projects the source illumination into an interferometer where the illumination is split into two separate illumination paths. The illumination from the two separate paths is recombined after exiting the interferometer and is projected onto a detector at an image plane where the interferogram is detected and subsequently analyzed.
In another embodiment, a point source is moved in a source plane in a manner so as to describe a circle of constant radius about the optical axis in less than the time it takes to expose one detector frame. In this way a xe2x80x9cvirtualxe2x80x9d ring shaped source is established.
In yet another embodiment, the radius of the source ring (produced directly or virtually) is changed dynamically, in either continuous or stepwise fashion, while the detector senses the interferogram. As the ring radius changes, the phase of the interferogram changes in a predictable way, providing the ability to modulate the interferometric phase in a manner required by phase-shifting or phase-stepping interferometry applications.
In an aspect of the invention, a point source is moved laterally with respect to the optical axis, producing an interferometric phase change, while simultaneously the interferometric phase is shifted by another phase modulator so as to keep the interferometric phase nominally constant. In this way, the interferometric phase is held constant while the illumination direction changes, substantially smearing out the effects of coherent artifacts. The additional phase shift can be accomplished with a conventional mechanical translator translating one of the interferometer surfaces or by tuning the illumination wavelength.
In another aspect of the invention, one surface of a transparent flat is measured in the presence of interference from the other surface by moving a point source laterally with respect to the optical axis, producing an interferometric phase change, while simultaneously the interferometric phase is shifted by another phase modulator so as to keep the interferometric phase of the interferogram produced by the desired surface of the flat nominally constant during the time it takes to expose one detector frame. In this way, the interferometric phase from the desired surface is held constant while the interferometric phase from the other surface changes substantially, washing out the interference fringes from the undesired surface.